National Instruments 320682C

Chapter 5 RS-232 Library

Hardware Handshaking

The SetCTSMode function enables hardware handshaking. For hardware handshaking to work,

two conditions must exist. First, the serial devices must follow the same or similar hardware

handshake protocols (they must use the same lines for the handshake and assign the same

meanings to those lines). Second, the serial cable connecting the two devices must include the

lines required to support the protocol. Because no single well-defined hardware handshake

protocol exists, resolve any differences between the LabWindows/CVI hardware handshake

protocol and the one your device uses.

Most serial devices primarily rely on the CTS and RTS lines to perform hardware handshaking,

and the DTR line is used to signal its online presence to the other device. Some serial devices

also may use the DTR line for hardware handshaking similarly to the CTS line. The

SetCTSMode function has two different modes to handle either case.

This SetCTSMode function employs the following line behaviors for each mode.

Note: Under UNIX, changes to the DTR line have no effect on the communication port.

LWRS_HWHANDSHAKE_OFF

• The RTS and DTR lines are raised when opening the port and lowered when closing the port.

Data is sent out the port regardless of the status of CTS.

Note: Under Windows, the SetComEscape function can be used to change the value of

the RTS and DTR lines.

LWRS_HWHANDSHAKE_CTS_RTS

• When the PC is the receiver:

– If the port is opened, the library raises RTS and DTR.

– If the input queue of the port is nearly full, the library lowers RTS.

– If the input queue of the port is nearly empty, the library raises RTS.

– If the port is closed, the library lowers RTS and DTR.

• When the PC is the sender:

– The RS-232 library must detect that its CTS line is high before sending data out the port.

LWRS_HWHANDSHAKE_CTS_RTS_DTR

• When the PC is the receiver:

– If the port is opened, the library raises RTS and DTR.